1. Technical Field
The present invention relates to a technique useful in the fermentation industry, that is, a method for efficiently producing an L-amino acid, especially hydrophobic amino acids, and L-threonine and L-glutamic acid, by fermentation using a microorganism. Hydrophobic L-amino acids are useful as components of nutrient mixtures for medical care. Furthermore, these amino acids are useful in various ways as additives for animal feed and reagents in the drug industry and chemical industry. Moreover, L-phenylalanine is also useful as a raw material in sweeteners. Furthermore, L-threonine is useful for animal feed, and L-glutamic acid is widely used as a raw material in seasonings, etc.
2. Background Art
L-amino acids are industrially produced by fermentation using coryneform bacteria or Enterobacteriaceae which are able to produce amino acids. Bacterial strains isolated from the nature or artificial variants of such strains, recombinant strains with recombinantly enhanced L-amino acid biosynthesis enzymes, and so forth are used to improve the productivity.
Examples of strains which are able to produce the hydrophobic amino acid L-tryptophan by fermentation include strains with enhanced activity of one of more enzymes such as anthranilate synthase, phosphoglycerate dehydrogenase, and tryptophan synthase (WO94/08031), and strains transformed with the tryptophan operon (Japanese Patent Laid-open (Kokai) Nos. 57-71397 and 62-244382, U.S. Pat. No. 4,371,614).
Furthermore, for L-glutamic acid fermentation, Japanese Patent Laid-open No. 63-214189 discloses a technique for increasing L-glutamic acid-producing ability by amplifying genes encoding glutamate dehydrogenase (gdh), isocitrate dehydrogenase (icdA), aconitate hydratase (acnA, acnB), and citrate synthase (gltA).
Furthermore, for L-threonine fermentation, Japanese Patent Laid-open No. 2001-346578 discloses an L-threonine-producing bacterium in which the aspartokinase III gene (lysC), aspartate semialdehyde dehydrogenase gene (asd), aspartokinase I gene (thrA), homoserine kinase gene (thrB), and threonine synthase gene (thrC), which are all encoded by threonine operon, are enhanced.
L-amino acid productivity has been considerably increased by the aforementioned breeding of microorganisms or improvement of production methods. However, in order to respond to further increases in demand in the future, the development of methods which provide more efficient production of hydrophobic L-amino acid at a lower cost are still necessary, and therefore, still represent a need in the art.
A method of performing fermentation by crystallizing the L-amino acid which accumulates in the culture medium is known (Japanese Patent Laid-open No. 62-288, European Patent Publication No. 1078989). The purpose of this method is to maintain a constant concentration of the L-amino acid in the culture medium by precipitating the L-amino acid into the culture medium.
Furthermore, a method of producing L-glutamic acid by using a microorganism which can produce L-glutamic acid by precipitation of L-glutamic acid is disclosed (U.S. Pat. No. 6,905,819).
Furthermore, as a method for crystallizing a hydrophobic L-amino acid, the method of purifying an L-amino acid by using a water-soluble cellulose derivative is known (Japanese Patent Publication (Kokoku) No. 5-76463). However, there have been no reports to date of a method of performing fermentation with precipitation of L-amino acids, wherein the L-amino acid precipitates in the medium and productivity of the L-amino acid is improved by adding a polymer such as a water-soluble cellulose derivative to the medium.